Liquid Crystalline Blue Phases Research Articles

Reduced graphene oxide (RGO) enriched polymer network for highly-enhanced electro-optic performance of a liquid crystalline blue phase

A reduced graphene oxide enriched polymer network enhances the operation voltage, response time, and hysteresis of a liquid crystalline blue phase.

Asymmetric band gap shift in electrically addressed blue phase photonic crystal fibers.

In this paper, we present electrooptic experiments on photonic crystal fibers filled with a liquid crystalline blue phase. These fibers guide light via photonic band gaps (PBGs). The blue phase is isotropic in the field-off state but becomes birefringent under an electric field. This leads to a polarization dependent shift of the PBGs. Interestingly, the effect on the PBGs is asymmetrical: while the short wavelength edges of the PBGs shift, the long wavelength edges are almost unaffected. By performing band gap and modal analyses via the finite element simulations, we find that the asymmetric shift is the result of the mixed polarization of the involved photonic bands. Finally, we use the band gap shifts to calculate effective Kerr constants of the blue phase.

Effects of thiophene-based mesogen terminated with branched alkoxy group on the temperature range and electro-optical performances of liquid crystalline blue phases

ABSTRACTA series of symmetrically thiophene-based bent-shaped molecules with branched terminal was synthesised and characterised. Then, their effects as dopants on the blue phase (BP) range of the chiral nematic liquid crystal (N*LC) host were investigated. It was found that the bent-shaped dopants with branched terminal had better miscibility in LC host than the bent-shaped dopants with straight terminal, and contributed to induce BP and enhance the BP temperature range, with the maximum BP temperature range about 20.4°C. Besides, the electro-optical (E-O) performances of the blue phase liquid crystal doped with Th-BC6 (a bent-shaped dopant with the widest induced BP range in N*LC) were also explored. It was found that the drive voltage reduced first and then increased with the doping amount of Th-BC6 increasing. When the doping amount of Th-BC6 was about 15 wt%, the hysteresis could be strikingly reduced.

The effects of asymmetric bent-shaped compounds on the temperature range and electro-optical performances of liquid crystalline blue phases

Asymmetric bent-shaped molecules improve the temperature range and electro-optical performances of liquid crystalline blue phases.

A monodomain-like liquid-crystalline simple cubic blue phase II

In this work, a systematic approach to controlling the orientation of a liquid-crystalline simple cubic blue phase (BPII) was carried out using a conventional surface rubbing treatment method. A more uniform lattice plane alignment in BPII was produced than in the liquid-crystalline-body-centered blue phase (BPI) using rubbed polyimide (PI) films on glass substrates. A single-color monodomain-like BPII was achieved regardless of the rubbed PI substrate pretilt angle. Additionally, single-color monodomain-like BPIIs with three elementary colors (red, green, and blue) were prepared, simply by changing the percentage of the chiral dopant.

Dielectric properties and molecular motions of liquid crystal molecules in 4-(2-methylbytyl)phenyl 4-(4-octylphenyl)benzoate liquid crystal having blue phase (CE8)

Abstract Blue phase liquid crystals exhibit unique properties which are used in the new type of display. A blue-phase liquid crystal display was first presented commercially by Samsung in 2007. The blue-phase-three-color pixel display eliminates the need for color filters. This type of display uses blue-phase multi-component liquid crystal. Considering the one-component systems, it turns out that they are stable only in a very narrow range of temperatures between the isotropic and the chiral nematic phase (about 1 K). In 2005, a wide temperature range BP multi-component system was reported by researchers from the University of Cambridge. There are still several unsolved problems left. One of them is chemical stability and reliability. Therefore, the knowledge of molecular dynamics of blue phase liquid crystal is a prerequisite for understanding of blue-phase multi-component system. Understanding the molecular dynamics of a single component liquid-crystalline blue phase system can facilitate the solution of these problems. We present the molecular dynamics investigation of 4-(2-methylbytyl)phenyl 4-(4-octylphenyl)benzoate (CE8), which may be a good candidate to form materials suitable for blue-phase liquid crystal displays.

Uniform Alignment of Liquid Crystalline Cubic Blue Phase II via Rubbing Treatment

The uniform alignment of liquid crystalline cubic blue phase II (BPII) has been achieved using a conventional surface rubbing treatment. Through comparison of thick and thin cells of assembled rubbed glass, which were covered with commercially available polyimide (PI), we confirmed that rubbed surfaces play an important role in the fabrication of monodomain PBII. These well-aligned cubic BPII materials can be promising for photonics devices.

Effects of Host Liquid Crystal Composition on the Stability of Liquid Crystalline Blue Phases

The temperature range of blue phase liquid crystal can be extended through the use of polymer stabilization. In this work, the mixing condition for the host liquid crystal mixture containing E8, PE-5CNF, and CPP-3FF; and the monomer UCL-011 and the chiral dopant NYC-22133L was investigated. Typically, the operating temperature for displays should be in the range of −40 to +80°C. The chiral dopant and monomer tend to lower the clearing temperature; thus, the host LC should have a high BP-Iso clearing temperature. In this work, optimization of the ratio of the host liquid crystal with respect to the monomer was carried out. It was found that a ratio of 5:3:2 for E8: PE-5CNF: CPP-3FF gave a polymer stabilized blue phase of ∼50.4°C. Driving voltage and electro-optic hysteresis was also investigated, yields a conclusion that a trade-off between driving voltage and hysteresis exists for the PSBPLC mixture that was investigated.

Optically binary liquid crystalline blue phases induced by one-armed cholesterol-linked azobenzene molecules.

A series of one-armed cholesterol-linked azobenzene molecules named CholXAzo with different spacers were synthesized, in which Chol6Azo was found to have induced blue phases (BPs) with a concentration of 4.0 wt%. Under irradiation of 385 nm UV light with a density of 15.0 mW cm(-2), photo-responsive behaviour of the 4.0 wt% Chol6Azo doped sample named B3 shows a sensitive temperature dependence, which means that at 38.0 °C a phase transition from BPs to the isotropic phase is induced; however, at 33.0 °C, this phase transition does not take place. Results from the research show that the optically binary phase transition behaviour of B3 is sensitive to the isomerization degree of Chol6Azo, which is closely related to the stability of the BP structure and there is a critical isomerization degree of 13.7% for the phase transition of the B3 liquid crystals. Further POM observation shows that the liquid crystal samples doped with different concentrations of Chol6Azo have an increasing transition temperature for photo-induced phase transition from the BP to the isotropic phase along with the increasing concentration of Chol6Azo, which are found to have the same changing tendency with phase transition temperature from the isotropic phase to BPs and a phase diagram is made to map the optically binary behaviour of Chol6Azo doped blue phase liquid crystals. At last, a simple pattern with the BP and the isotropic phase arranged at an interval was made in this optically binary liquid crystalline blue phase under a suitable photomask.

An equilibrium double-twist model for the radial structure of collagen fibrils

Mammalian tissues contain networks and ordered arrays of collagen fibrils originating from the periodic self-assembly of helical 300 nm long tropocollagen complexes. The fibril radius is typically between 25 to 250 nm, and tropocollagen at the surface appears to exhibit a characteristic twist-angle with respect to the fibril axis. Similar fibril radii and twist-angles at the surface are observed in vitro, suggesting that these features are controlled by a similar self-assembly process. In this work, we propose a physical mechanism of equilibrium radius control for collagen fibrils based on a radially varying double-twist alignment of tropocollagen within a collagen fibril. The free-energy of alignment is similar to that of liquid crystalline blue phases, and we employ an analytic Euler-Lagrange and numerical free energy minimization to determine the twist-angle between the molecular axis and the fibril axis along the radial direction. Competition between the different elastic energy components, together with a surface energy, determines the equilibrium radius and twist-angle at the fibril surface. A simplified model with a twist-angle that is linear with radius is a reasonable approximation in some parameter regimes, and explains a power-law dependence of radius and twist-angle at the surface as parameters are varied. Fibril radius and twist-angle at the surface corresponding to an equilibrium free-energy minimum are consistent with existing experimental measurements of collagen fibrils. Remarkably, in the experimental regime, all of our model parameters are important for controlling equilibrium structural parameters of collagen fibrils.

Thermal phase transition behaviours of the blue phase of bent-core nematogen and chiral dopant mixtures under different boundary conditions.

We have investigated dramatic changes in the thermal phase transition of a liquid-crystalline (LC) blue phase (BP) consisting of bent-core nematogen and chiral dopants under various boundary conditions during cooling from the isotropic phase. Blue phase III, which has arbitrary morphological characteristics, between two glass plates goes to blue phase I, which has a body-centred cubic structure, when the sample is placed on the glass as a droplet form. These two distinctive phase transition behaviours were directly observed by polarized optical microscopy as well as selective reflection spectroscopy, in which the phase transition between BPs started from the air/LC interface. We determined that this transition resulted from the ordered double-twisted cylinders near the air-boundary by confocal laser-scanning microscopy.

The effect of graphene on liquid-crystalline blue phases

The stabilization of liquid-crystalline blue phases is recently attracting considerable interest because of the envisioned applications in fast optical displays and tunable photonic crystals. We report on the effect of surface-functionalized graphene nanosheets on the blue phase range of a chiral liquid crystal. Calorimetric and optical measurements, reproducible on heating and cooling, demonstrate that the resulting soft nanocomposite exhibits an increased blue phase temperature stability range for a minute concentration of dispersed graphene. The impact is stronger on the ordered, cubic structured blue phase I. These findings suggest that anisotropic nanoparticles may be of great usefulness for stabilizing blue phases.

Liquid crystalline blue phase in mixtures of fluorinated compounds with positive and negative dielectric anisotropy and its electro-optic performance

Dependence of a blue phase temperature range on a chiral dopant and cyano compound addition as well as character of nematic base mixture was investigated. Base nematic mixtures of fluorinated compounds, one with positive and the other one with negative dielectric anisotropy, were chosen. Various fluoro compounds, chiral compounds and cyano compound were added in order to evaluate the relationship between blue phase stability and molecular structure of components. The stability of a blue phase in selected mixtures has been extended by polymer network formation. Fundamental electro-optic properties such as Kerr constant, operating time and switching voltage of a polymer-stabilised blue phase with positive and negative dielectric anisotropy are given, too. It has been confirmed that blue phase temperature range and its electro-optic performance strongly depends on molecular structure of mixture components as well as chiral dopants and their helical twisting power. Specifically, it was found that the cyano compound addition causes destabilisation of blue phase in mixtures consisted of fluorinated compounds.

Effect of anisotropic MoS2nanoparticles on the blue phase range of a chiral liquid crystal

Liquid-crystalline blue phases are attracting significant interest due to their potential for applications related to tunable photonic crystals and fast optical displays. In this work a brief theoretical model is presented accounting for the impact of anisotropic nanoparticles on the blue phase stability region. This model is tested by means of high-resolution calorimetric and optical measurements of the effect of anisotropic, surface-functionalized MoS2 nanoparticles on the blue phase range of a chiral liquid crystal. The addition of these nanoparticles effectively increases the temperature range of blue phases and especially the cubic structure of blue phase I.

Liquid‐Crystalline Blue Phase Laser with Widely Tunable Wavelength

A lasing peak shift of more than 100 nm is realized due to the large shift of a photonic bandgap of a liquid-crystalline blue phase.

Effects of symmetrically 2,5-disubstituted 1,3,4-oxadiazoles on the temperature range of liquid crystalline blue phases: a systematic study

Four series of symmetrically 2,5-disubstituted 1,3,4-oxadiazoles with different lateral substituents and terminal alkoxy chain length were synthesised and characterised. Then, all these bent-shaped compounds were separately doped into the blue phase (BP) liquid crystal host and their effects on the BP range of the host were investigated, which shows that the BP ranges varied greatly with the difference in the structure of 2,5-disubstituted 1,3,4-oxadiazoles and their doped concentration. Moreover, in order to explore the influence of the inherent molecular geometry and properties of the bent-shaped molecules on the stability of BPs, the molecular structures of all the compounds were studied by using the theory of density functional theory, and the optimised structural parameters of the molecules were calculated at the B3LYP/6–31G* level.

Stabilization of the liquid crystalline blue phase by the addition of short-chain polystyrene

This paper examines the liquid crystalline blue phases of mixtures of a readily available chiral system (consisting of cholesteryl nonanoate and cholesteryl benzoate) with various proportions of a low molecular weight and low polydispersity polymer. By observing the textures of samples using polarizing optical microscopy as they are heated from the chiral nematic through the blue phase and into the isotropic liquid phase, the temperature range where the blue phase of these mixtures is stable is measured as a function of polymer volume fraction for several molecular weights of polystyrene and poly(methyl methacrylate). The mixtures with polystyrene showed a blue phase stability range of up to 12 K, one of the largest measured on heating, and this range varied smoothly as a function of polymer volume fraction. The dependence of the range on the polymer volume fraction and molecular weight could be captured well by a theoretical model based on the Flory–Huggins and Maier–Saupe theories.

Material design for blue phase liquid crystals and their electro-optical effects

Liquid-crystalline blue phases have attracted a considerable amount of attention because of their hierarchical phase structures with optical isotropy and because of their application to electro-optical devices. The driving forces for the frustrated liquid-crystalline phases differ from those for a conventional liquid-crystalline phase composed of rod-like molecules. Classical molecular design is inapplicable to blue phase liquid crystals. From perspectives of display applications, blue phases exhibit some revolutionary features. They obviate surface alignment, and provide submillisecond response time and an isotropic dark state. Nevertheless, critical problems remain, such as high driving voltage and hysteresis in the voltage–transmittance curve. After reviewing recent progress in stabilizing blue phases, we describe their electro-optical performance. Some advanced concepts in material design widen the blue phase temperature range. Furthermore, development of materials and device structures have prepared blue phase LCD for use in next-generation liquid-crystal displays. Blue phases are no longer a special liquid crystal exhibiting an extremely narrow temperature range.

Stabilization of blue phases by hydrogen-bonded bent-shaped and T-shaped molecules featuring a branched terminal group

The stabilization of liquid crystalline blue phases (BPs) from novel hydrogen-bonded (H-bonded) bent-shaped and T-shaped assemblies (HBAs) has been described for the first time in this study. Different kinds of H-bonded bent-shaped and T-shaped molecules were assembled, and then used to broaden the BP temperature range of liquid crystal (LC) samples. The results demonstrate that the temperature range of BPs is significantly influenced by HBA dopants, in which the widest BP temperature range was 15.1 °C. Moreover, the lower temperature of BPs is very close to room temperature, which is of benefit to the application of BPs in LC displays. Additionally, according to the experimental analysis and molecular simulations, the physical mechanism of the BP stabilization by bent-shaped and T-shaped molecules is mainly related to the molecular geometry and is not dependent on the LC phase. This study has developed a new and facile strategy to widen the BP temperature range from the aspect of the molecular design.

Dendron‐Stabilized Liquid Crystalline Blue Phases with an Enlarged Controllable Range of the Photonic Band for Tunable Photonic Devices

AbstractLiquid crystalline blue phases (BPs) show excellent potential for application in tunable photonic devices because they possess the unique optical property that the selective 3D Bragg diffraction in a visible wavelength region can be continuously shifted using an electric field. A new approach to simultaneously extend the wavelength range of field‐induced Bragg diffraction shift and the temperature range of thermodynamically stable BPs is critically needed. Here, a new BP material system is shown using a dendron molecule to extend simultaneously the two BP ranges. One is the temperature range of thermodynamically stable BPs, which is expanded from 2.1 to 4.6 °C. The other is the reversible maximum shift range of Bragg wavelength on the electric field, which is extended from 85 to 109 nm. The physical mechanism of the dendron‐stabilizing effect in BPs is discussed in terms of elastic property and orientational order of liquid crystal molecules.